This invention relates to the preparation of lithographic sheets or plates of aluminum or aluminum alloys, particularly to an improved anodization process which enables the production of products displaying superior performance and, at the same time, the simplification of the operation through the elimination of treatment steps, which heretofore were considered essential or desirable.
It has been customary in known processes to provide an anodizing step, following the roughening of the plate surface by a suitable mechanical, chemical, or electrolytic treatment. However, it has been considered necessary or desirable in the past, following the anodization step, to apply a sealing or other supplementary treatment, as with an aqueous silicate or dichromate solution, to provide a suitable surface for the proper reception and adequate adhesion of the subsequently applied light-sensitive plate coating layer. This layer may contain any of a large variety of known light-sensitive compounds, such as various diazo or other nitrogen-containing organic compounds, which react to be converted to water-insoluble oleophilic image areas after being irradiated with light transmitted through a negative and subsequent development. After the removal of the unexposed non-image areas of the plate surface, such areas are hydrophilic and only the image areas are receptive to the lithographic printing inks, thus enabling the reproduction of the desired printed images. That is, only the oleophilic image areas of the plate are receptive to the printing ink, and the resulting ink image may readily be transferred to a suitable roller, and then printed in its final form.
Such a sequence of essential steps inevitably imposes severe requirements for durability and for strong adhesion of the printing layer to the lithographic plate in order to enable the obtainment of excellent printed reproductions in very large numbers with the use of each plate. While various proposals have been made to effect improvements of such nature, they have not been sufficiently effective and most have increased the complexity of treatment, and therefore the cost, to an undue extent.